Grivet, Chlorocebus aethiops
GRIVET
Chlorocebus aethiops
Geographic Distribution and Habitat
Grivet monkeys live in the savannas, scrublands, or mixed grassland-woodland habitats of northeastern Africa, specifically in the countries of Djibouti, Eritrea, Ethiopia, South Sudan, and Sudan. They are found close to water sources, like rivers, but adapt well to different environments—even urban areas. Grivets are also sometimes referred to as African green monkeys.
Many monkeys in the genus Chlorocebus have a slight greenish tinge to their fur, which inspired their genus name. In Greek, “chloro” means green, and “cebus” means long-tailed monkey. The taxonomy of grivets is a matter of debate, as some taxonomists consider grivets (Chlorocebus aethiops) to be a larger group with many subspecies. The currently accepted classification, based on anatomical and genetic studies, considers the grivet to be a monotypic species, meaning it does not have any subspecies.
In areas where grivets overlap with other Chlorocebus species, there are cases of hybrid offspring of both species.
Until fairly recently, all members of the genus Chlorocebus, including grivets, green monkeys, tantalus monkeys, malbroucks, and vervets themselves, were referred to as “vervets.” There is still a great deal of overlap in data and resulting confusion. The species within the Chlorocebus genus are quite similar in appearance yet have some distinguishing characteristics, especially as relates to their geographic ranges, food sources, threats, and lifespans.
These recent taxonomic changes could mean that previous research that referred to Chlorocebus aethiops did not necessarily apply to the northeastern African grivets themselves but to the more southern vervet monkeys.
Size, Weight, and Lifespan
The average weight of an adult grivet is about 11 pounds (5 kg) with males being visibly heavier at 7–17 pounds (3.1–6.4 kg) than females who range between 3–5 pounds(1.5–4.9 kg). Grivets are medium-sized monkeys with a head-to-body length of 16–23 inches (42–60 cm) for males and 12–19 inches (30–50 cm) for females. Their tails are longer than their bodies, measuring 18–22 inches (46–66 cm).
These monkeys are long-lived, with one individual living for 32 years in captivity. In the wild, they live up to 17 years. Some studies have revealed that, as grivet monkeys become old, they develop conditions like Alzheimer’s in humans.
Appearance
Grivet monkeys have wiry, grizzled fur that is sandy-olive on their backs and mostly white on their fronts. Their faces are black, with white whiskers that extend from their chins and merge with their white chest fur. They have large, amber-brown eyes surrounded by pale cream-colored skin. Compared to more arboreal or tree-dwelling species, grivets have shorter torsos and limbs, which makes them better at running on the ground.
Grivets have cheek pouches, where they can temporarily store food while they travel. This allows the monkeys to carry as much food as they can and eat later once they have gathered all their food sources.
Diet
Grivets are opportunistic omnivores and feed on grasses, roots, flowers, fruits, insects, small birds, and even rodents. The natural habitat of grivets is highly variable with few trees. This makes the availability of foods like fruits and flowers unpredictable, so grivets need to depend on other food sources for nutrition. They adapt well to changing environments and food availability. In areas in which their natural habitats have been converted to farms, grivets shifted to eating crops, to the displeasure of the farmers.
Behavior and Lifestyle
Food availability and diet have a significant impact on the behaviors and ecology of the grivet monkey. Because their habitat is variable with few trees and vegetation, grivets do not always know where their next meal will come from. When food is concentrated in one area, such as fruits on a tree, grivets become protective of these food sources. In such cases, grivets, particularly females, establish dominance hierarchies or pecking orders in which older or larger monkeys have first access to the best food sources and will attack grivets who intrude on their meals. On the other hand, when food is spread out over an area, grivets do not show dominance over others regarding food. When food is scarce, grivets focus on finding and eating what they can, rather than wasting energy on defending food sources that are difficult to find.
It turns out that grivets are good learners and problem solvers. They have found different ways to adapt to their environment. They eat different types of food and have learned to raid crops and avoid getting caught by farmers. One research group found that grivets that spent time around humans and their various fabricated contraptions were able to figure out how to open strange boxes and objects (like artificial fruit) better than grivets that had no exposure to humans.
Grivets are diurnal and most active in the morning and early evening before sunset. They are considered semi-terrestrial because they spend most of their time on the ground (they do not really have an option because there aren’t many trees in their habitat), but they are dextrous and can also climb and jump into trees when they need to. At night they sleep in tree canopies.
Mostly, they walk quadrupedally (on all four limbs) on the ground and even use all four limbs to scratch themselves and groom each other. Some researchers take this as a sign that grivets have a lot of ectoparasites, such as ticks and fleas. As primates that spend a lot of time on the ground and in the dirt, they are more likely to pick up parasites.
- Grivets are highly adaptable monkeys that feed on fruits, seeds, insects, and even small birds.
- They are agile, capable of walking on all fours and jumping in trees on their two hind legs.
- When baby grivets are born they have dark fur and pink faces, which is completely different from the adults.
Grivets have to travel to find food and—when they do find a food source, such as a fruiting tree—they spend most of their time eating as much as possible. Between eating food, most adults take time to rest or groom each other while youngsters play. While they forage on the ground, they spend nights resting in trees, as it is safer from predators and parasites.
Grivets live in multi-male, multi-female troops with slightly more females than males in the troop. There are usually more babies and juveniles than adult males. However, the ratio of males to females and juveniles depends on the habitat and quality of food available to the grivets. More dependable food sources will result in larger troops and more juveniles. Adult females focus their energy on getting enough food and taking care of their young, while adult males mostly take care of protecting the troop from neighboring troops. Males and females seem to have their own hierarchies regarding which individuals are dominant over others.
Grivets, like closely related vervet monkeys, have complex vocal signals that they use to communicate with each other. Vocalizations are usually accompanied by intense gestures and facial expressions. The most commonly heard grivet sounds are loud alarm calls that they make when they spot danger. However, these alarm calls are not as elaborate and long-lasting as those of vervet monkeys. When fights break out between troops, grivets will scream and flail their arms to intimidate the intruding grivets. Chattering noises are made when communicating with family members to make contact.
There is no particular breeding season and all of the males will mate with most of the adult females in the troop. Females reach sexual maturity around 3 years old; males take longer, and mature when they are 5 years old.
When a female is ready to mate, she will signal males through scent hormones (pheromones) and present herself to the males, who will then mate with her. After a gestation period of 3–5 months, a mother gives birth to a single offspring that weighs about 0.7 pounds (314 g). Grivet babies are born with black fur and pink faces. Over the first three to four months, their faces darken and their coats lighten to the adult olive-gray coloring.
Grivet infants are totally dependent on their mothers for food and protection. Infants cling to their mother’s belly for at least the first week of their lives. As the baby becomes more independent, they will stray from the mother and interact with other troop members. If there is a sign of danger, the mother will swoop in to carry the baby to safety. Males do not take much interest in young grivets.
Grivet females stay in their natal troops while males will leave once they mature into adults. Therefore females are considered to be the core of the troop who forms strong bonds with the young and more permanent adult males.
As omnivores, grivets occupy multiple niches in their environment—or, in other words, they do many jobs in their habitat. They eat the leaves, bark, fruit, and the gum of acacia trees and they also spread seeds that help grow the acacia population. Grivets eat insects, which helps control the many thousands of beetles, termites, and other insects that thrive in the savanna and rocky habitats. The grivet’s adaptive ability to switch between food types allows them to use their environment in a way few other animals can.
Grivets are also a food source for predators such as leopards, lions, and eagles. The many ways in which grivets fit into their habitat means that a severe decline in the population will affect many parts of the environment.
The International Union for Conservation of Nature lists grivets as Least Concern (IUCN, 2022), appearing on the IUCN Red List of Threatened Species.
The grivet monkey is not yet at risk of extinction because it is widely distributed and its population decline is not yet a threat. On average, in their habitat, there are about 1–9 grivet monkeys per 0.3 square mile (1 sq km). In some regions of Ethiopia, grivet density is as high as 73 individuals per 0.3 square mile (1 sq km).
Habitat loss and conversion of natural areas are the main cause of population decline in grivets. As grivets adapt to human developments, the number of human-monkey conflicts increases when monkeys try to steal food or inhabit villages and cities. The unfortunate consequences of these conflicts are that “problematic” grivet monkeys are usually hunted and removed from human settlements.
Grivets are considered pests because they raid crops and most farmers will kill grivets on sight. Hundred of grivets are hunted or trapped every year and an unfortunate result of this is a rise in the number of grivet orphans that struggle to survive without their mother and troop mates.
Grivets are sometimes kept as pets and are also extensively used in laboratory and medical research.
The grivet monkey is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
Across the continent, grivets are given protection to prevent trade and illegal hunting.
There are some organizations that work on rehabilitating captured or orphaned grivets and releasing them into the wild. The best chances of conserving grivets are setting aside protected areas where they can thrive away from human communities, and conflict management programs where farmers are given tools to manage grivets that raid crops in a non-lethal manner.
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Written by Acima Cherian, July 2023